As a mage, I sometimes stumble upon other mages who are questing in the same area. My first encounter with some 'competitive' mages began quite early, polymorphing mobs I was killing and generally being rude. I didn't know how to counter this at first, but later I started using a wand to conserve mana when I quest (when a mob was low on hp, for example) and realized it was an easy way of taking away the polymorph effect (polymorph typically takes a second or so to take the regen into effect). Polymorph cast-time; 1.5. Wand cast-time; 1.6. Perfect! It saved mana, I could cast instant casts in between cooldowns and wand attacks, it took away the polymorph effect almost immediately, and it also made the mob temporarily stop attacking me. This is how I learned to counter other mages polymorphing my quest mobs. Hope that helps!
